The presence of random involuntary limb movement superimposed on purposeful limb movement is an abnormal condition that afflicts hundreds of thousands of patients suffering from a variety of diseases. Many tremor patients are disabled by these involuntary movements due to the fact that the amplitude of these movements is large enough to degrade or obscure voluntary movement attempted by the patient. Cerebral palsy patients suffering from athetosis may also be disabled by their involuntary limb movement. Chorea is another such condition.
In each of these cases, patients typically try to overcome the disability imposed by the involuntary movements of a particular limb either by steadying its motion using an unafflicted limb, by jamming the afflicted limb against the body so as to restrict its vibration, or even by having another person grasp the limb to steady its motion. Drug therapies and surgery have been attempted with limited effectiveness and considerable risk for the patient.
However, in the past ten years or so, a small number of assistive devices, i.e., orthoses, have been developed experimentally that are meant to selectively suppress the random involuntary movements. These devices are based on the experimental observation that significant reduction of the involuntary movements can be achieved by the application of viscous damping to the afflicted limb or body segment.
One such device is a one degree-of-freedom (DOF) orthosis with an electronically-controlled magnetic particle brake used to retard limb motion (See Dunfee, D. E., "Suppression of Intention Tremor by Mechanical Loading", M.S. Thesis, M.I.T. Department of Mechanical Engineering, February 1979). This device, meant primarily for conducting experiments at the wrist, prevents the patient from performing whole-arm functional activities, since limb motion is rigidly constrained in the remaining DOF's.
Another prior art device is a 2 degree-of-freedom joystick meant to serve as a control interface to electrical devices (such as powered wheelchairs) while applying a resistive load to the limb. This system cannot be used for whole-arm movements and is not meant as a general purpose functional orthosis.